Hot on the heels of a study that found that simply describing a wine as more expensive made it taste better comes the discovery that the same advice is more likely to be followed if it costs more.
The study was led by organisational psychologist Francesca Gino and is covered by the BPS Research Digest:
Dozens of students were asked questions about American history and received small cash prizes for correct answers. The students were either given the option of receiving advice on the correct answers, or advice was imposed on them. Sometimes this advice was free; other times it was paid for out of the students’ winnings.
Crucially, the advice always came from the same source – in the form of the answer that a student from a pilot session had given to the same question – so the quality of advice was held constant regardless of whether it was free or paid for.
Throughout the study, the participants took more account of advice they had paid for than advice they were given free, even though it was made clear to them that the advice was of the same quality. A final study showed the students took even more account of advice if it was made more expensive.
The full text of the study is freely available online as a pdf, although if you’re not convinced of the findings I’m sure Dr Gino would be happy to supply an additional copy for a small fee.
Link to BPSRD on the behavioural value of expensive advice.
Mind Hacks 
The “sunk cost fallacy” is a great hypothesis, but it’s also likely that students who were *already* willing to follow advice (i.e. those who didn’t have a clue about American history) were also willing to pay for it. They may be thinking that if they fail, they’ll get nothing; but if they pay for advice (and hence have a higher chance of answering correctly), at least they’ll get a fraction. Those really desperate would also accept a higher price.
So it may not be a matter of “Well, I paid for it, it *must* be worth something,” but instead of clinging to their only hope at getting any money at the end of the experiment. Shortly put: being smart.